1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor and an electrophotographic image forming apparatus. More particularly, the invention relates to a two-layered type electrophotographic photoreceptor including a naphthalenetetracarboxylic acid diimide derivative having a nitro group as an electron transporting material in a charge generating layer including a charge generating material. The photoreceptor has improved electrostatic properties such as photosensitivity and residual potential. The present invention also relates to an electrophotographic image forming apparatus including the electrophotographic photoreceptor.
2. Description of the Related Art
Electrophotography is widely used in laser printers, photocopiers, facsimile machines, LED printers, CRT printers, and laser electrophotographs, and the like. An electrophotographic photoreceptor includes a photosensitive layer formed on an electrically conductive substrate and can be in the form of a plate, a disk, a sheet, a belt, or a drum, etc. In an electrophotographic photoreceptor, a surface of the photosensitive layer is uniformly and electrostatically charged, and then the charged surface is exposed to a pattern of light, thus forming an image. The light exposure selectively dissipates the charge in the exposed regions where the light strikes the surface, thereby forming a pattern of charged and uncharged regions. This pattern is referred to as a latent image. Then a wet or dry toner is supplied in the vicinity of the latent image, and toner droplets or particles are deposited in either the charged or uncharged region to form a toner image on the surface of the photosensitive layer. The resulting toner image can be transferred and fixed to a suitable final or intermediate receiving surface, such as paper, or the photosensitive layer can function as the final receptor for receiving the image.
Electrophotographic photoreceptors can be classified into two types. The first is a type having a structure including a charge generating layer (CGL) comprising a charge generating material (CGM), a binder resin, and a charge transporting layer (CTL) comprising a binder resin and a charge transporting material (primarily, a hole transporting material (HTM)). In general, the two-layered type electrophotographic photoreceptors are used in the fabrication of negative (−) type electrophotographic photoreceptors. The other type is a single-layered type photoreceptor in which a binder resin, a charge generating material, a hole transporting material (HTM), and an electron transporting material (ETM) are contained in a single layer. In general, the single-layered type photoreceptors are used in the fabrication of positive (+) type electrophotographic photoreceptors.
The charge generating material is provided for the purpose of generating charge carriers (that is, holes and/or electrons) upon exposure. The purpose of the charge transporting material is to receive at least one type of the charge carriers and transport them through the charge transporting layer in order to facilitate the discharge of the surface charges on the photoreceptor.
The amount of the charge generating material in the charge generating layer of the two-layered type electrophotographic photoreceptor needs to be large in order to obtain an electrophotographic photoreceptor with high photosensitivity. However, if the amount of the charge generating material is too large, the stability of the coating slurry for forming the charge generating layer may be degraded, thus degrading the coating quality of the charge generating layer. Also, the adhesive force of the charge generating layer and the charge generating layer and an electrically conductive substrate and the adhesive force of the charge transporting layer may be degraded. On the contrary, if the amount of the charge generating material is low, the stability of the coating slurry for forming the charge generating layer, the coating quality of the charge generating layer, the adhesive force of the charge generating layer and the electrically conductive substrate and the adhesive force of the charge generating layer and the charge transporting layer may be improved, but the electrostatic properties may be radically degraded such that the photosensitivity of the electrophotographic photoreceptor may decrease and the residual potential may increase.
Also, regardless of the amount of the charge generating material in the charge generating layer, electron transportation in the charge generating layer is not good, thereby adversely affecting the electrostatic properties of the electrophotographic photoreceptor such that the photosensitivity of the electrophotographic photoreceptor tends to be low and the residual potential thereof tends to be high. In particular, since the charges are mainly generated in the upper portion of the charge generating layer, degradation of the electrostatic properties due to poor electron transportation occurs more significantly when the thickness of the charge generating layer is increased for high photosensitivity.
U.S. Pat. Nos. 5,547,790, 5,571,648, and 5,677,094 respectively disclose an electrophotographic photoreceptor, for solving the above described problems,
U.S. Pat. No. 5,547,790 discloses an electrophotographic photoreceptor including at least a charge generating layer and a charge transporting layer that are sequentially stacked on an electrically conductive substrate. The charge generating layer includes a charge generating material selected from the group consisting of azo pigments, perynone pigments, and squaraines and a polymeric charge transporting material. The charge transporting layer includes a polymeric charge transporting material. The polymer charge transporting material in the charge generating layer is selected from a polysirylene, a polymer having a hydrazone structure on the main bone and/or side chain thereof, and a polymer having a tertiary amine structure on the main bone and/or side chain thereof. The polymer charge transporting material in the charge transporting layer is a polymer having a polysirylene, a polymer having a hydrazone on the main bone and/or side bone thereof, and a polymer having a tertiary amine structure on the main bone and/or side chain thereof.
U.S. Pat. No. 5,571,648 discloses an electrophotographic imaging member comprising a support substrate having a two electrically conductive ground plane layer comprising a layer comprising zirconium over a layer comprising titanium, a hole blocking layer, an adhesive layer comprising a copolyester film forming resin, an intermediate layer in contact with the adhesive layer, where the intermediate layer comprises a film forming carbazole polymer, a charge generating layer comprising perylene or phthalocyanine particles dispersed in a film forming a polymer binder blend of polycarbonate and carbazole polymer, and a hole transporting layer, wherein the hole transporting layer is substantially non-absorbing in the spectral region at which the charge generating layer generates and injects photogenerated holes but is capable of supporting the injection of photogenerated holes from the charge generating layer and transporting the holes through the charge transporting layer.
U.S. Pat. No. 5,677,094 discloses an electrophotographic photoconductor comprising an electroconductive support and a photoconductive layer formed on the electroconductive support and including a charge generating layer and a charge transporting layer, wherein the charge generating layer comprises a first polymeric charge transporting material having an ionization potential of 6.0 eV or less, and the charge transporting layer comprises a charge transporting small molecule and a binder.
The electrophotographic photoreceptors disclosed in the above U.S. Patents tried to improve electrostatic properties by further incorporating hole transporting material besides the charge generating material to the charge generating layer. However, an electrophotographic photoreceptor with improved electrostatic properties is still required.